1. Field of the Invention
The invention relates to excimer and molecular fluorine lasers, and particularly having an unstable resonator configuration for improving one or more beam parameters such as suppressing spectral fluctuations.
2. Discussion of the Related Art
A typical lithography laser resonator includes a plane-plane resonator configuration, and more generally, a stable cavity. For example, U.S. Pat. No. 5,095,492, 5,150,370 and 5,559,816, which are hereby incorporated by reference, describe lithography laser resonators having stable cavities, wherein the ""492 patent describes a linear resonator and the ""370 and ""816 patents describe polarization coupled resonators.
FIG. 1 schematically shows an illustrative laser resonator containing an active medium AM and two plane resonator mirrors M1 and M2. The active medium AM has a length d and it is recognized in the present invention that the active medium AM is characterized by a time-spatial dependent refractive index n(x,y,t)=n0+xcex94n(x,y,t). The refractive index change can be created, e.g., by non-uniform temperature change in the medium due to non-uniform power deposition. It is further recognized in the present invention that this effect may play a significant role for high power lasers such as high repetition rate lasers. For instance, if a beam passes through the active medium, it sees different optical path lengths s over the cross section due to the spatially-dependent optical path length s(x,y,t)=dxc2x7n(x,y,t). the time-spatial dependent path length s(x,y,t) results in changes of the beam divergencies and/or beam direction. For m round trips through the cavity of a plane-plane resonator such as that illustrated schematically at FIG. 1, the beam sees the total path length s=2xc2x7mxc2x7n(x,y,t). That means that in a plane-plane cavity the wavefront distortion effect is amplified. This is illustrated by the solid line shown at FIG. 2. In view of this, it is recognized in the present invention that it would be advantageous to provide a resonator configuration that serves to suppress these wavefront distortions.
Narrow band excimer lasers having cylindrical unstable resonators A have been described in U.S. Pat. Nos.5,946,337 and 5,970,082, which are hereby incorporated by reference. The curvatures of the resonator optics (i.e., cylindrical mirrors and/or cylindrical lenses) are described as being oriented in one plane, while the dispersion plane of the narrow band optics (e.g., an intracavity grating) is aligned parallel or perpendicular to that unstable resonator plane in the ""082 and ""337 patents, respectively. It is desired according to the present invention to provide narrow band excimer or molecular fluorine laser radiation with low beam divergency in one dimension.
Excimer lasers are applied in the art of photolithography for production of integrated circuits. Achromatic imaging optics for this wavelength region are difficult to produce. For this reason, line-narrowed excimer laser radiation is used for photolithography in order to prevent errors caused by chromatic aberration. It is recognized herein that the bandwidths for different imaging systems tabulated below in Table 1 may substantially represent acceptable bandwidths for suppressing these chromatic aberrations for the laser wavelengths 248 nm (KrF laser), 193 nm (ArF laser), and 157 nm (F2-laser).
Current lithography lasers work with repetition rates up to 2 kHz. To get higher throughput, it is recognized herein that repetition rates should be increased to 4 kHz or higher, e.g., 10 kHz or more. The averaged power in the laser cavity will rise as the repetition rate is increased, e.g., by a factor 2 to 4 or more, connected with a very high thermal load on all intracavity optical components, and especially of the narrow band optics. This results in wavefront distortions due to thermally induced changes of the refractive index resulting in time dependent variations of the laser spectrum and of near and far field intensity distributions. Additional variations of the refractive index in the cavity may be created by discharge fluctuations in the discharge chamber. Intracavity fluctuations of the beam can lead to changes in significant output beam parameters such as beam size, beam position, divergence, beam pointing, etc., outside of the cavity, also. One term that may be used to describe this effect is xe2x80x9cbeam dancingxe2x80x9d. Beam dancing may result, e.g., in energy fluctuations in the image plane due to apertures in the beam line to the stepper or in the light path in the stepper itself. It is recognized in the present invention that this effect should be suppressed as much as possible. It is also recognized in the present invention that one way to correct the beam is by using adaptive optical techniques (see R. K. Tyson, Principles of Adaptive Optics, Academic Press (1991), which is hereby incorporated by reference).
In view of the above, an unstable laser cavity in each of orthogonal cross section beam axis directions is provided to suppress angular, directional and/or other laser beam parameter instabilities.
Also in view of the above, a laser resonator is provided including one or more optics for magnifying the beam in each of orthogonal cross sectional beam axis directions for suppressing fluctuations in one or more output beam parameters.
In particular, an excimer or molecular fluorine laser system is provided including a discharge chamber filled with a gas mixture including at least a halogen-containing molecular species and a buffer gas, multiple electrodes within the discharge chamber and connected to a pulsed power supply circuit for energizing the gas mixture and a resonator for generating an output beam. The resonator includes the discharge chamber, at least one line-narrowing optic and resonator reflecting optics disposed at either end of said resonator. The resonator reflecting optics are configured to form an unstable resonator in each of orthogonal cross-sectional beam axis directions for suppressing fluctuations in one or more output beam parameters.
An excimer or molecular fluorine laser system is also provided including a discharge chamber filled with a gas mixture including at least a halogen-containing molecular species and a buffer gas, multiple electrodes within the discharge chamber and connected to a pulsed power supply circuit for energizing the gas mixture and a resonator for generating an output beam including the discharge chamber and resonator reflecting optics at either end. The system further includes a first intracavity optic for narrowing a bandwidth of the output beam and configured to magnify the beam in a first cross-sectional beam axis direction and a second intracavity optic configured to magnify the beam in a second cross-sectional beam axis direction angularly offset from the first cross-sectional beam axis direction. The first and second intracavity optics are configured for suppressing fluctuations in one or more output beam parameters.
An excimer or molecular fluorine laser system is further provided including a discharge chamber filled with a gas mixture including at least a halogen-containing molecular species and a buffer gas, multiple electrodes within the discharge chamber and connected to a pulsed power supply circuit for energizing the gas mixture and a resonator for generating an output beam including the discharge chamber and resonator reflecting surfaces at either end. The system further includes a first intracavity optic for narrowing a bandwidth of he output beam, a second intracavity optic configured to magnify the beam in a first cross-sectional beam axis direction and a third intracavity optic configured to magnify the beam in a second cross-sectional beam axis direction angularly offset from the first cross-sectional beam axis direction. The first, second and third intracavity optics are configured for suppressing fluctuations in one or more output beam parameters
An excimer or molecular fluorine laser system is further provided including a discharge chamber filled with a gas mixture including at least a halogen-containing molecular species and a buffer gas, multiple electrodes within the discharge chamber and connected to a pulsed power supply circuit for energizing the gas mixture and a resonator for generating an output beam including the discharge chamber and resonator reflecting surfaces at either end. The system further includes an intracavity optic for narrowing a bandwidth of the output beam and configured to magnify the beam in a first and a second orthogonal cross-sectional beam axis directions for suppressing fluctuations in one or more output beam parameters.
An excimer or molecular fluorine laser system is also provided including a discharge chamber filled with a gas mixture including at least a halogen-containing molecular species and a buffer gas, multiple electrodes within the discharge chamber and connected to a pulsed power supply circuit for energizing the gas mixture and a resonator for generating an output beam including the discharge chamber and resonator reflecting surfaces at either end. The system further includes a first intracavity optic for narrowing a bandwidth of the output beam and a second intracavity optic configured to magnify the beam in a first and a second orthogonal cross-sectional beam axis directions for suppressing fluctuations in one or more output beam parameters.
An excimer or molecular fluorine laser system is also provided including a discharge chamber filled with a gas mixture including at least a halogen-containing molecular species and a buffer gas, multiple electrodes within the discharge chamber and connected to a pulsed power supply circuit for energizing the gas mixture and a resonator for generating an output beam including the discharge chamber and resonator reflecting optics at either end. The system also includes at least one optic for narrowing a bandwidth of the output beam, a first intracavity optic configured to magnify the beam in a first cross-sectional beam axis direction, and a second intracavity optic configured to magnify the beam in a second cross-sectional beam axis direction angularly offset from the first cross-sectional beam axis direction, for suppressing fluctuations in one or more output beam parameters.
An excimer or molecular fluorine laser system is further provided including a discharge chamber filled with a gas mixture including at least a halogen-containing molecular species and a buffer gas, multiple electrodes within the discharge chamber and connected to a pulsed power supply circuit for energizing the gas mixture, and a resonator for generating an output beam including the discharge chamber and resonator reflecting surfaces at either end. The system further includes at least one optic for narrowing a bandwidth of the output beam, a reflective optic configured to magnify the beam in a first cross sectional beam axis direction and an intracavity optic configured to magnify the beam in a second cross sectional beam axis direction angularly offset from the first cross sectional beam axis direction. The reflective and intracavity optics are configured for suppressing fluctuations in one or more output beam parameters.
A method for suppressing fluctuations in one or more beam parameters of an excimer or molecular fluorine laser system is also provided including energizing a gain medium of the laser system by applying a pulsed discharge for generating the beam, narrowing a bandwidth of the beam by interacting the beam with one or more line-narrowing optical elements and magnifying the beam in orthogonal cross-sectional beam axis directions by interacting the beam with one or more magnifying optical elements for suppressing the fluctuations in the one or more output beam parameters.
A method for suppressing fluctuations in one or more beam parameters of an excimer or molecular fluorine laser system is also provided including energizing a gain medium of the laser system by applying a pulsed discharge for generating the beam, narrowing a bandwidth of the beam by interacting the beam with one or more line-narrowing optical elements and magnifying the beam intracavity in orthogonal cross-sectional beam axis directions by interacting the beam with one or more magnifying optical elements for suppressing the fluctuations in the one or more output beam parameters.
A method for suppressing fluctuations in one or more beam parameters of an excimer or molecular fluorine laser system is also provided including energizing a gain medium of the laser system by applying a pulsed discharge for generating the beam, narrowing a bandwidth of the beam by interacting the beam with one or more line-narrowing optical elements and magnifying the beam intracavity in a first cross-sectional beam axis direction and magnifying the beam in a second cross-section beam axis direction angularly offset from the first cross sectional beam axis direction by interacting the beam with one or more magnifying optical elements for suppressing said fluctuations in said one or more output beam parameters.